1. Field of the Invention
The present invention relates to a method for producing an ethylene-vinyl alcohol copolymer (EVOH) resin, and to a method for producing EVOH resin pellets by cutting the EVOH resin produced according to the former method.
More precisely, the invention relates to a method for producing an EVOH resin, which comprises introducing an alcohol-containing EVOH solution into an apparatus, and contacting the solution with water in the apparatus, thereby letting alcohol along with water out of the vessel and transferring the resulting aqueous EVOH composition into an extruder, kneading it in melt therein, and extruding it out of the extruder; and to a method for producing EVOH resin pellets by cutting the EVOH resin produced according to the former method.
2. Description of the Related Art
Ethylene-vinyl alcohol copolymer is a useful polymer material having good oxygen barrier properties, aroma retentiveness, oil resistance, antistatic properties and mechanical strength. It is widely used for films, sheets, containers, etc. One general method of producing EVOH comprises saponifying an ethylene-vinyl ester copolymer that is obtained through polymerization of ethylene with a vinyl ester of a fatty acid, such as vinyl acetate or the like, in an alcohol-containing organic solvent, in the presence of a saponification catalyst.
For post-treating the alcohol solution of EVOH obtained through saponification Japanese Patent Publication No. 38634/1972 discloses a method of producing EVOH pellets which comprises adding water to a methanolic solution of EVOH optionally-concentrated so that no EVOH deposit is formed therein in order to prepare a methanol-water mixed solution of EVOH that contains from 15 to 45% by weight of EVOH, then extruding it into strands in water or in a methanol-water mixed solution having a lower methanol concentration than the EVOH solution, at 50° C. or lower, and thereafter cutting the strands. The application states that the concentration of methanol in the coagulating bath preferably is between 10 and 50% by weight. The application further states that the pellets thus obtained are porous and can be readily washed with water to remove the saponification catalyst residue and that the pellets are easy to handle in the subsequent washing and drying step.
Regarding the process of adding water to a methanol solution of the EVOH after saponification, Example 1 of Japanese Patent Laid-Open No. 90927/1999 describes a method of producing a completely transparent, uniform methanol/water solution of EVOH, which comprises adding an aqueous methanol solution having a water content of 62.5% by weight to a methanol solution of EVOH having an EVOH content of 30% by weight, under an azeotropic condition at 100° C. to 110° C. under a pressure of 3 kg/cm2G so as to remove methanol until the EVOH content of the resulting EVOH solution increases up to 40% by weight.
However, in the method of producing EVOH pellets by coagulating an EVOH solution that contains a large amount of methanol, the alcohol often vaporizes in the step of coagulation. The alcohol vapor worsens the working environment and injures the health of the workers, and, in addition, it has negative influences on the area around the working site. Therefore, improving the method is desired.
Where EVOH having an ethylene content of less than 20 mol % or a degree of saponification of less than 95% is produced according to the method as above, EVOH strands are difficult to form in the coagulating bath since the coagulation rate of EVOH in the bath is low. As a result, the EVOH strands, even though formed, are often miscut and are often contaminated with fine powder. Stable production of EVOH pellets by this method is difficult. Moreover EVOH having an ethylene content of 20 mol % or more and a degree of saponification of 95% or more is poorly coagulated or is difficult to handle, when it is coagulated in strands at an increased coagulation rate through an increased number of nozzles so as to increase the production efficiency, and, as a result, its strands are often miscut and stable production of its pellets is not easy.
In addition, the EVOH pellets obtained in the conventional methods contain alcohol, catalyst residues, alkali metal salts, and therefore require washing to remove them. To increase the washing speed, elevating the temperature of the washing liquid will be effective. However, if the temperature is elevated too high, the pellets will fuse as they contain alcohol.
Various methods are known for molding EVOH pellets. In general, EVOH pellets are molded in extrusion or injection molding apparatus. To mold in this manner, the starting material of EVOH resin pellets is first fed into the hopper of an extruder where, in general, the temperature of the resin melt should not be lower than 200° C. However EVOH containing no additive will readily degrade if molded in melt in this condition, often forming fish eyes or hard spots in the products to lower the quality of the products.
A known method for improving the long-run workability of EVOH and for preventing EVOH moldings from having surface defects such gels or hard spots that worsen the outward appearance of the moldings is to add minor components such as acid substances and/or metal salts to the EVOH. For example, Japanese Patent Laid-Open No. 66262/1989, discloses an EVOH composition which contains from 0.0005 to 0.05% by weight of a Group 2 metal salt, from 0.002 to 0.2% by weight of an acid having a pKa of at least 3.5 and a boiling point of not lower than 180° C., and from 0.01 to 0.2% by weight of an acid having a pKa of at least 3.5 and a boiling point of not higher than 120° C., and which has specific flow properties.
The following are examples of the method for obtaining EVOH pellets containing minor components as above: (1) A method of spraying an aqueous solution of minor components on EVOH pellets followed by mixing them in a Henschel mixer and drying them (Japanese Patent Laid-Open No. 12108/1980). (2) A method of adding powdery minor components to EVOH pellets followed by blending them in dry in a super-mixer (Japanese Patent Laid-Open No. 34148/1982). (3) A method of dipping EVOH pellets in an aqueous solution of minor components followed by dewatering and drying them (Japanese Patent Laid-Open No. 66262/1989). (4) A method of controlling the water content of EVOH pellets to fall between 20 and 80% by weight followed by contacting the EVOH pellets with an aqueous solution of at least one compound selected from boron compounds, acetates and phosphoric acid compounds (WO99/05213).
In the methods (1) and (2), however, the minor components added to the EVOH pellets could not be uniformly dispersed. In addition, the amount of the minor components to be added is difficult to control, and products of stable quality are difficult to obtain. In methods (3) and (4) it is possible to control the content of the minor components in the EVOH pellets by controlling the concentration of the solution of the minor components.
As mentioned above, pellets of EVOH having an ethylene content of less than 20 mol % and those having a degree of saponification of less than 95% are difficult to produce stably, and they often form crumbs. Even EVOH having an ethylene content of at least 20 mol % and EVOH having a degree of saponification of at least 95% often form crumbs when they are processed into strands at high speed through an increased number of nozzles for enhancing the production efficiency. However, when the crumbed EVOH is processed according to the method of (3) or (4), the minor components are difficult to mix uniformly with it, and, as a result, products of stable quality are difficult to obtain.
From the viewpoint of the production process, the conventional method of dipping EVOH pellets in a treating solution that contains an acid substance and/or a metal salt is troublesome as it requires a treating bath or column for dipping the pellets therein. In the treating solution used in such a treating bath or column, the acidic substance and/or the metal salt are reduced as they are consumed by EVOH treated with it. Therefore, the used solution shall be discarded as waste, or may be recovered and recycled. For the latter, however, a suitable amount of fresh acidic substance and/or metal salt must be added to the used solution. Discarding the waste of the used solution as such is undesirable for environmental protection, and it requires some additional equipment for waste treatment. On the other hand, in case where the used solution is recovered and recycled, it also requires additional equipment for adding fresh acidic substance and/or metal salt to the used solution and additional equipment for removing impurities such as typically oligomers from the used solution for obtaining products of stable quality. The conventional methods involve serious problems in that they require large-scale treating equipment and long treating time and therefore the production costs increase.
For melt-extruding wet EVOH, proposed are a method of extruding it in melt at a specific level of energy (Japanese Patent Laid-Open No. 58500/1999); and a method of extruding it in melt under specific control of the cylinder temperature of the extruder used (Japanese Patent Laid-Open No. 58501/1999). However, these references are silent on the problem of how to control the water content of EVOH in melt extrusion.